The invention concerns a light barrier or grid with transmitting and receiving units longitudinally arranged at intervals in a strip-like housing, each of them including an optoelectronic element that is spaced apart from at least one optical function element.
Light grids are extensively used for the recognition and monitoring of objects on conveyor belts, for baggage inspection on conveyor belts, for controlling elevator doors, for protecting body parts at machine tools, for access control of people and vehicles, etc. The light grids have spaced-apart transmitting units arranged in a strip-like housing and receiving units arranged in a strip-like housing with a corresponding spacing. The transmitting units emit light beams, typically infrared light, which impinge on the receiving units. Interruptions of the light beams are detected and evaluated.
The transmitting units and the receiving units each include an optoelectronic element and an optical function element placed at a distance from the former in the direction of the beam. At the transmitting unit, an optoelectronic transducer emits light which is focused onto the corresponding receiving unit by the optical function element, for example a lens or a diaphragm. The receiving unit has a corresponding optical function element which directs the light at an optoelectronic transducer.
The optoelectronic elements and the corresponding optical function elements of the individual transmitting and receiving units are mounted in a housing, and they must be aligned relative to each other and relative to the housing. For this, it is known to assemble the particular optoelectronic elements and the corresponding optical functions elements, for example, by means of a tube, to form a subassembly in which the optoelectronic elements and the optical function elements are properly oriented and aligned in respect to each other. The subassemblies are installed in the housing, and they then have to be oriented in relation to the housing. The mounting of the optoelectronic elements and the optical function elements to form the subassembly and the mounting of the subassembly in the housing result in an addition of the individual assembly tolerances.
It is known to configure the housing as a U-shaped profile in which the subassemblies are inserted from the open front end and clipped in place. Such a U-shaped housing has a relatively low torsional rigidity. Improving the torsional rigidity, especially in the case of an extruded plastic profile, requires larger material cross-sections and/or expensive materials.
It is also known to use an enclosed, hollow profile for the housing. In this case, the subassemblies have to be longitudinally pushed into the housing from an open end of the profile. This is cumbersome, especially when the housing is relatively long and makes it difficult to accurately orient and align the subassemblies relative to the housing. If an extruded profile is used, machining is also necessary, which increases manufacturing costs.
The use of trough-shaped plastic housings is also known. They enable an easy mounting of the subassemblies. Such housings, however, require additional metal reinforcements to provide the needed stability and adjustability.